1. Field of the Invention
The present invention relates to a sheet feeding device and an image forming apparatus, and more particularly to a sheet feeding device that separates and feeds sheets by blowing air on the sheets.
2. Description of the Related Art
Conventionally, as an image forming apparatus such as a copying machine or a printer, an image forming apparatus has been proposed that includes an air-blowing sheet feeding device that floats sheets by blowing air onto them so that the sheets are conveyed by suction to a suction conveyance belt.
Japanese Patent Application Laid-Open No. 2007-276910 discusses a sheet feeding device capable of reliably feeding sheets whose edge portion on the downstream side in the sheet conveyance direction has a curl that bends upward (hereinafter referred to as “upward curl”).
The sheet feeding device discussed in Japanese Patent Application Laid-Open No. 2007-276910 will now be described based on FIG. 11A. Sheet surface control is performed based on position detection of a topmost sheet by a sheet surface detection mechanism 49 so that a distance between a suction conveyance belt 21 and the topmost sheet is within an appropriate range S1. At the edge portion of the sheet on the downstream side in the sheet conveyance direction, which is near an air blowing unit 30, sheet surface control is performed so that the distance of the topmost sheet from the suction conveyance belt 21 is within the range S1. Consequently, the blowing of air by the air blowing unit 30, and the suction and conveyance by the suction conveyance belt 21 are appropriately performed.
This sheet feeding device has a detection unit 61, which is connected to a sensor lever 52 that turns a sensor unit (including a first sheet surface sensor 54 and a second sheet surface sensor 55) ON/OFF. The detection unit 61 extends toward the downstream side in the sheet conveyance direction. Sheet surface control is performed by the detection unit 61 contacting the topmost surface of the sheets stacked on an elevating tray, to position the topmost stacked sheet within an appropriate range by turning the sensor unit ON/OFF to raise or lower the elevating tray.
As illustrated in FIG. 11A, at the edge portion of the sheet on the downstream side in the sheet conveyance direction, for an upward curl sheet, sheet surface control is performed by the detection unit 61 contacting the topmost sheet so that the distance of the topmost stacked sheet from the suction conveyance belt 21 is within the appropriate range S1. Consequently, the blowing of air by the air blowing unit 30 and the suction and conveyance by the suction conveyance belt 21 can be appropriately performed, thus enabling the sheets to be reliably separated and fed.
However, in the sheet feeding device discussed in Japanese Patent Application Laid-Open No. 2007-276910, there is still room for improvement when the edge portion on the downstream side in the sheet conveyance direction has a curl that bends downward (hereinafter referred to as “downward curl”).
As illustrated in FIG. 11B, in the sheet feeding device discussed in Japanese Patent Application Laid-Open No. 2007-276910, sheet surface control is performed so that, even for a sheet that has a downward curl, the topmost sheet is positioned within the range S1. However, in this case, the position where the detection unit 61 contacts the topmost sheet is a position away from the air blowing unit 30 on the upstream side in the feeding direction. Consequently, the distance between the topmost sheet and the suction conveyance belt 21 at the edge portion of the sheet on the downstream side in the sheet conveyance direction that is near the air blowing unit 30 becomes S2, so that the topmost sheet is substantially out of the appropriate range S1.
Consequently, when trying to convey the sheets by suction, the suction can be incomplete, so that the leading edge of the fed sheets may hit a downstream guide, thereby causing problems such as paper jamming. Further, it can become impossible to convey the sheet by suction because the gap between the sheet and the suction conveyance belt is too wide.